On my Mac (27" iMac with Apple keyborad with numeric keypad and arrow keys), (OS X 10.8.5) and LV 2013 (32-bit) the scan codes are left = 123, right = 124, down = 125, and up = 126. Afer changing the array in altenbach's VI to those values, the VI runs.
All the 2-arrow combinations work. Left+Right+Up and Left+Right+Down work. Up+Down works. No three arrow combination with both Up and Down works and the 4-arrow combination does not work.
Running with execution highlighting on (patience, please) shows that the event structure does not fire when the third or fourth key in the non-working combinations is pressed. This makes me think it may be an OS issue - not generating the interupt for the keypress.
Lynn